Detection of base contaminants in gas samples

ABSTRACT

A detection system for detecting contaminant gases includes a converter, a detector, a primary channel for delivering a target gas sample through the converter to the detector, and at least two scrubbing channels for delivering a reference gas sample through the converter to the detector. Each of the scrubbing channels includes a scrubber for removing basic nitrogen compounds from the reference gas sample, while the primary channel preferably transmits the target gas sample without scrubbing. The converter converts gaseous nitrogen compounds in the target gas sample to an indicator gas, such as nitric oxide (NO), and a control system directs the flow of a gas sample among the primary channel and the scrubbing channels. In accordance with one aspect of the invention, the basic-nitrogen-compound concentration can be measured by comparing the concentration of the indicator gas detected in the reference sample with the detected indicator-gas concentration in the target sample. The use of multiple scrubbing channels enables the detection to operate continuously since each scrubber can be alternately purged while another is scrubbing.

BACKGROUND OF THE INVENTION

The invention relates to the detection of base contaminants in a gassample, especially amine contaminants, and to systems employing suchdetection, including semiconductor fabrication systems and systems forfiltering gases for semiconductor fabrication and other processes thatrequire uncontaminated atmospheres of high quality.

A particular purpose of the invention is to reliably measure lowconcentrations of airborne base contaminants in a semiconductormanufacturing environment that may adversely affect base-sensitivephotolithographic processes being employed.

In semiconductor manufacturing it has been found desirable to detectairborne basic compounds such as normal methyl pyrrolidinone (NMP) andammonia. Such contaminants may interfere, for instance, with aphotolithography process used in semiconductor fabrication. The basecontaminant may react with protons produced as a result of exposure of aphotoresist layer to light. This can interfere with proper exposure anddevelopment and can harm the yield of the process and the rate ofproduction of the semiconductor wafers.

For this reason, semiconductor manufacturers have sought to measure andcontrol the concentration of airborne molecular contamination during thecritical steps of the photolithography process that are sensitive to it.A detecting instrument specific to the detection of NMP and a detectinginstrument specific to the detection of ammonia have been employed insemiconductor manufacturing facilities to monitor the atmosphericquality in the vicinity of production tools.

To understand the novel aspects of the invention it is useful to mentionsome detection techniques that have been used in other contexts.

For study of combustion processes or atmospheric pollution, some havedeveloped processes for measuring the total fixed gaseous nitrogenspecies, including NH₃, NO, NO₂, HCN and organic amines in gaseousmixtures. The process involves catalytic conversion at elevatedtemperature of all fixed nitrogen species to NO, followed bychemiluminescent measurement of the resulting NO concentration.

For detection of ammonia, NO and NO_(X), machines have been made thatemploy an ammonia scrubber or absorber coupled with a thermal/catalyticconverter with or without a molybdenum catalyst. For instance, in oneinstrument for stack gas analysis, a diluted sample is directed by avalve to alternatively flow through or past an absorber thatspecifically removes ammonia. The alternating samples proceed along acommon line through a thermal converter to a chemiluminescent detectorthat operates in the 650-750 millibar range. By subtracting signals, theammonia concentration can be calculated.

Another aspect of the invention relates to the use of air filters forthe ambient air in semiconductor manufacturing. To avoid harm to theprocess from NMP or ammonia, semiconductor manufacturers have usedchemical filters to remove the contaminants. These filtering systemsemploy filter stages within an enclosure, the filter media of each stagebeing penetrable by air with acceptable pressure drop. As air flowsthrough the filtering system, unwanted contaminants are retained on thechemically active surface of the various stages of the filter system. Aproblem associated with such filtering systems has been to accuratelypredict the remaining life of the filter so that the filter media can bechanged at appropriate times with minimal disruption to the use of theexpensive production facility. In the case of semiconductor fabricationfacilities, typically, filter life has been estimated by measuring theconcentration of ammonia in the air flow associated with the filtersystem.

DISCLOSURE OF THE INVENTION

The measurement of ammonia, exclusive of other basic contaminants, isunsatisfactory in photolithographic processes that are affected by lowconcentrations of any basic contaminant gas. One process that issensitive to low levels of any basic contaminant gas ischemically-amplified deep-ultraviolet (DUV) photoresist processing.Typically, most or all of such basic contaminants that can affect theprocess include nitrogen. Measurement of total fixed-nitrogen species isnot applicable, however, because many of the fixed-nitrogen species(e.g., HCN, NO, NO₂) are not basic in nature and do not affect theprocess.

None of the techniques mentioned above have suggested the concept of thepresent invention of measuring—in a single, non-specific reading—alow-level concentration of multiple basic nitrogen compounds in a gassample exposed to photolithographic processes and the like.

The invention is based in part on the realization that semiconductormanufacturing and certain other processes, which are recognized to besensitive to NMP, ammonia, or other basic nitrogen compounds, are infact sensitive to the total proton-bonding capability of all nitrogenousbase contaminants present, regardless of the specific identity of thenitrogenous base contaminants. According to the invention, rather thandetermine the presence and concentration of each individual contaminantby a separate detector, it is realized that important advantages can beobtained by providing a detector that provides a single reading that isstoichiometrically related to the aggregate proton-bondingcharacteristic of various nitrogenous base contaminants that may bepresent in the monitored air. In this way, a “totalbasic-nitrogen-compound detector” is provided.

As explained further below, what is recognized to be of use is ameasurement of the totality of those multiple basic-nitrogen-compoundcontaminants in the gas sample that can adversely affect the processbeing monitored. For instance, currently-employed DUV photolithographyprocesses are sensitive to both strong and weak bases; hence, accordingto the present invention, all airborne basic nitrogen compounds aremeasured down to low concentration levels. In other cases, where theprocess is sensitive only to bases greater than a certain pK_(b), thenthe system is implemented, according to this idea of the invention, tomeasure the totality of the multiple basic nitrogen compounds within thepK_(b) range to which the process is sensitive, even atlow-concentration levels.

The present invention focuses specifically on a basic-nitrogen-compoundscrubber system as an important component of the entirebasic-nitrogen-compound detection system. While a single ion exchangebed can be provided in a channel to remove ammonia from a gas sampleflowing through the channel, this solution is not without drawbacks. Ofparticular concern is that ion exchange beds over time do noteffectively filter basic nitrogen compounds other than ammonia. Forexample, NMP, a higher-molecular-weight imide, can pass through a2-inch-deep, ½-inch-diameter scrubber in about 15 hours triggeringfalse-negative signals in a measurement of total basic nitrogencompounds. This is a major problem which affects the functionality andreliability of the instrument's output.

In a detection system of this invention, a primary channel and aplurality of scrubbing channels are connected to a detector. The term,“detector,” as used herein includes a single detection device/unit aswell as a plurality of detection devices/units. Each of the scrubbingchannels include a basic-nitrogen-compound scrubber. Downstream fromeach of the scrubbers is a converter which converts gaseous nitrogencompounds into an indicator gas and a detector for detecting theindicator gas. A primary channel also leads to the converter anddetector. As used, herein, the term, “converter,” refers either to asingle converter unit to which all channels are connected, in parallel,or to multiple converter units individually associated with a single ormultiple channels.

In a method of this invention, reference gas samples are passed throughfirst and second scrubbers to remove basic nitrogen compounds from thegas samples. The second scrubber is purged to remove reversibly-boundnitrogen compounds while a gas sample passes through the secondscrubber. The gas samples passing through either the first or secondscrubber then pass through the converter and detector. Alternately, atarget gas sample bypasses the scrubbers before delivery to theconverter and detector. The total concentration of basic nitrogencompounds in the gas samples is then determined on the basis of thedifference of the detected concentration of the indicator gas in thetarget gas sample and the detected indicator gas concentration in thereference gas sample.

In preferred embodiments of the system, the scrubbing channels areconnected in parallel, and the system includes purge systems coupled tothe scrubbers for purging reversibly-bound basic nitrogen compounds fromthe scrubbers. Preferably, the scrubbers include cation exchange media.A flow controller governed by a control system can be positioned toselectively control which of the scrubbing channels the sampled gas canflow through to the converter. The control system can be programmed totransfer the flow of the reference gas sample that reaches the detectorfrom a scrubbing channel with a contaminated scrubber to a scrubbingchannel with a purged scrubber and then to direct a purge gas throughthe contaminated scrubber. Preferably, the control system is programmedto transfer the flow of the reference gas sample away from a scrubbingchannel and to purge the scrubber of that scrubbing channel before aweak-base nitrogen compound can penetrate through the scrubber.Moreover, the purging process can alternate between scrubbers with theflow of gas samples to the converter preferably continuing substantiallywithout interruption. Finally, the control system can be programmed toalternately transfer the flow of a gas sample between the primarychannel and one of the scrubbing channels.

In accordance with one aspect of the invention, the system samples gasfrom a photolithography tool cluster for the fabrication ofsemiconductor wafers and is adapted to monitor the concentration ofbasic nitrogen compounds in the photolithography tool cluster. In aspecific form of this embodiment, the scrubbers includephotoresist-coated beads, the photoresist coating matching thephotoresist applied by the photolithography tool cluster.

Other preferred embodiments of the system include a pressure reducerlocated upstream of the detector and a vacuum pump located downstream ofthe detector. Further, a common converter and detection device can becoupled to the primary channel and the filtering channels. Furtherstill, scrubbers are preferably excluded from the primary channel, andat least one multi-way valve is preferably arranged for selecting whichof the channels the sampled gas may flow through to the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a DUV photolithography processing facilityemploying a contaminant detection system.

FIG. 2 is an enlarged schematic view of the filtration tower shown inFIG. 1.

FIG. 3 is a flow diagram illustrating the process of calibrating thedetection system of FIG. 1.

FIG. 4 is a flow diagram illustrating the continuous operation andcalibration of the embodiment of FIG. 1.

FIG. 5 is a flow diagram illustrating the monitoring and control of theprocessing tools and filtration system of the embodiment of FIG. 1.

FIG. 6 is a schematic view of a total basic-nitrogen-compound detectoras a mobile detection unit.

FIG. 7 is a schematic view of a photolithographic system in which atotal basic-nitrogen-compound detector is combined with a track.

FIG. 8A is a schematic view of a sample delivery train for totalbasic-nitrogen-compound detection that has a scrubber system to producean internal reference, and in which a pressure reducer is locateddownstream from the scrubber system.

FIG. 8B illustrates a basic-nitrogen-compound scrubber system.

FIG. 8C illustrates a sample delivery train with separate pressurereducers.

FIG. 8D illustrates a thermal converter useful in the system of FIG. 8A.

FIG. 8E is a schematic view of a detection system that includes threeseparate sample channels coupled to an NO detector.

FIG. 9 is a schematic view of a sample delivery train similar to that ofFIG. 8A in which an isolation valve is located upstream from a scrubbersystem.

FIG. 10 is a view similar to FIG. 8A of a system implemented to beimmune to variations in NO and other interferents at the sampling site.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a photolithography tool cluster is shown for the productionof semiconductor wafers. The cluster consists of two tools, a stepper 8and a track 9. A wafer processed by the cluster is coated withphotoresist in the track 9, transferred to the stepper 8 where thecoated wafer is exposed to ultraviolet radiation passing through areticle, and then transferred back to the track 9 where the exposedphotoresist is developed. Each of these tools 8, 9 is joined to aseparate clean air filtration system, 1 and 1 a, respectively. Eachfiltration tower comprises a metal enclosure 10 and a set of spacedapart chemically-active filter stages 12, 14, 16, 18 installed in serieswithin the enclosure. As depicted in FIG. 1, the air enters at 20, atthe top of the tower, the air being supplied from either outside thefabrication facility or from within the facility, or from within theclean room or the tool itself. This system and its operation are morefully described in U.S. patent applications Ser. No. 08/795,949, filedFeb. 28, 1997, now U.S. Pat. No. 6,096,267, and 08/996,790, filed Dec.23, 1997, the teachings of both of which are hereby incorporated byreference.

The filters are composed of chemically-active composite materials,typically nonwoven fabric media, to which are bound activated carbonparticles or ion exchange beads that have been treated to remove ammoniaand organic amines. The filter media is typically arranged as a set ofpleats in the enclosure. An example of such filter media is known by thetrademark, Vaporsorb™, produced by the Assignee, Extraction Systems Inc.of Franklin, Mass. U.S.A.

In another embodiment, a converter-detector is employed to monitorfilter performance of a filter deployed in either the make-up orrecirculation air supplying a clean room. In this case, theconverter-detector is employed in such a manner as to monitor totalbasic nitrogen compounds both upstream and downstream of a filterdeployed either alone or in series in the make-up or recirculation airsystem of the clean room.

In other implementations, different filter media are employed. Certainexamples include parallel trays of loose activated carbon particlesproduced by, e.g., Donaldson Company (Minneapolis, Minn. U.S.A.);extruded carbon blocks using a dry thermoplastic adhesive as the bindingagent as produced by, e.g., Flanders Filters (Washington, N.C., U.S.A.),KX Industries, and Peneer Industries; thin extruded carbon blocksmanifest as a fabric as manufacturing by, e.g., KX Industries; mediamade by the modification of the chemical properties of the fiberstructure as produced by, e.g., Ebara Corp. (Tokyo, Japan) and TakumaLtd.; and carbon fiber structures as produced by, e.g., Kondoh Ltd.; andcarbon particle sheet media produced by, e.g., Hoechst-Celanese.

As shown in FIG. 1, each filtration tower, 1 and 1 a, includes,respectively, an upstream sampling port 2, 2′, a downstream samplingport 4, 4′, and an intermediate sampling port 3, 3′. Sampling ports 8 aand 9 a are likewise provided for the stepper 8 and track 9,respectively. For each filter and tool combination, there is oneconversion module 6 (for the stepper 8) and 6′ (for the track 9). Theconversion modules 6 and 6′ are connected to a common, remotely-locatedNO detector 7.

In other embodiments, a single conversion module receives gas samplesfrom both tools 8 and 9 and delivers the converted samples to thedetector 7. In this case, the conversion module and the detector 7 canbe in the form of a Model 17 instrument, which is available from ThermoEnvironmental Instruments Inc. (Franklin, Mass. U.S.A.). Although theremaining description relating to FIG. 1 is generally directed to theillustrated embodiment, which includes a pair of conversion modules 6and 6′, a single conversion module can generally be used with equalsuccess.

A remotely-controlled manifold 5, 5′, is associated with each conversionmodule 6, 6′, respectively. Via respective sample lines, the manifolds5, 5′ direct to the conversion modules 6, 6′ respective samples from thetools 8, 9, from the inlet streams 2, 2′ of the filters, from the outletstreams 4, 4′ of the filters, and from the intermediate filter ports 3,3′, according to a sequence controlled by a computer 51. As shown inFIG. 8A, and as described in greater detail, below, the conversionmodules 6, 6′ include a pair of channels, A and B, through which theflow of sampled gas is alternated every 10 seconds, a scrubber system120 in channel A, a multi-way valve 122 for controlling and redirectingflow through channels A and B, a pressure reducer 118, and a thermalcatalytic converter 124 in which nitrogen compounds in a gas sample areconverted to nitric oxide (NO).

The converted samples are drawn from the conversion modules 6, 6′ to adetector 7 by a vacuum pump 22 located downstream of the detector 7. Thedetector 7 includes a reaction chamber where NO is oxidized to produceelectronically-excited NO₂ molecules and a photomultiplier tube whichmeasures photons emitted by the electronically-excited NO₂ molecules asthey return to their ground state.

In one embodiment, an impinger is employed to identify possiblecontaminants in the unconverted gas sample. The impinger consists of aglass or quartz tube holding a liquid. In this embodiment, a vacuum pumpand an associated calibrated flow controller are employed to draw thegas sample from one of the sample lines through the liquid to take agrab sample. The grab sample is then analyzed or subjected to real-timecolorimetric analysis, providing a quantitative assessment of thecontaminating basic nitrogen compounds in the sampled gas.

In an instrument associated with more than one conversion module, eachconverter-detector subsystem is considered as a single instrument, whichis calibrated independently of the other converter-detector subsystems.In the preferred embodiment illustrated in FIG. 1, there are twoconverter-detector subsystems: one subsystem serves the track 9 and itsair filter system 1 and the other subsystem serves the stepper 8 and itsair filter system 1 a. For calibration purposes, zero air is provided byfilter system 1 for the stepper subsystem and by filter system 1 a forthe track subsystem. By having the conversion module 6, 6′ near thesampling area, the length of the sampling lines exposed to basicnitrogen compounds is reduced, which increases the response time of thesystem.

To calibrate each converter-detector subsystem, two or more samples ofknown concentration of contaminants are provided to the instrument, asillustrated in FIG. 3. The instrument response is then compared with theknown concentrations, and a calibration curve is generated and eithermanually or electronically, through the software, associated with theinstrument to provide corrections to the instrument's response. Ingeneral, the instrument response over the concentration range remainsstable for an extended period. The instrument is sensitive, however, toloss of calibration, for reasons such as drift of the photomultiplier,and the curve must be shifted relative to the true zero reading as itvaries over time. Because in photolithography processing harmfulcontaminant concentrations are extremely low (on the order of 1 part perbillion (ppb) or lower), in preferred systems the zero calibration istypically performed on the order of once a month to assure the fidelityof the zero reading. In a particularly preferred implementation, thedetection system is arranged to operate continuously, as shown in FIG.4, whereby the system performs a total basic-nitrogen-compound detectionfor each of the sampling ports in turn and conducts two calibrationseach cycle, one with respect to each of the conversion modules 6, 6′with which the detector 7 operates.

Preferably, the zero air employed for calibration is provided by theoutlet ports 4 and 4′ of the filtration system (see FIG. 1). Theinstrument is then instructed to provide a zero reading for thecalibration sample. In the case that the difference between the totalbasic-nitrogen-compound reading for the outlet port 4 and the sample atthe intermediate port 3 is not greater than zero, the sample from theoutlet port 4 is employed to establish zero air. In another preferredembodiment, a sampling port located just preceding the last filter stageis employed to verify that the zero air from the output of the filterstack is in fact zero air. Also, in an alternative embodiment, abuilt-in, dedicated, zero air generator is employed. The generatorprovides zero air by either filtering the ambient air or by bubbling airthrough a liquid scrubber solution.

An external computer, preferably situated outside the clean room inwhich the tools are located, is employed to control the operation andmonitor the entire photolithography process. The software is customizedfor the required application. Performance data is provided to thecomputer to provide an archival database to be employed to give thecontamination history of the tool cluster.

Based on the particular ports being sampled, the software employed inthe operation of the instrument determines which converter-detectorsubsystem is to be calibrated and the appropriate source of zero air forcalibration purposes. The software also designates which calibrationcurve to employ. As the detection system is calibrated and the new zeroreadings are determined, the calibration curves are adjustedaccordingly.

In a desired application, control instrumentation, as illustrated inFIG. 5, monitors the performance of the filtering system and the levelof contamination at the track and stepper tools. Should a reading fromeither the stepper or track exceed a predetermined threshold, an alarmis enabled and the process is immediately shut down. However, by use ofthis detection system, the occurrence of such an emergency can normallybe avoided.

As shown in FIG. 5, the filtering system is continuously monitored inreal time, as follows. The sample at the inlet to the filter system,over time, provides a quantitative history of the input ofbasic-nitrogen-compound contaminants to the filter. By using samplesdrawn from the intermediate position along the filter system as well asfrom the outlet of the filter stack, and by measuring the difference inconcentration levels from these locations, one of the following steps istriggered. If the difference is zero (condition green) and the totalamine or Bronstead base concentration at the tool is within operatinglimits, then the operation continues without interruption. When thedifference is greater than zero, the difference is compared with apredetermined threshold. If the threshold is not exceeded (conditionyellow), operation continues but a filter replacement is scheduled. Ifthe threshold is exceeded, or if the total basic nitrogen compounddetected at the tool exceeds operating limits (condition red), theoperation is immediately shut down.

In another embodiment, there are three or more conversion modulesremotely located at various locations in the fabrication facility. Oneconversion module is employed to monitor the general conditions in theclean room; a pair of conversion modules is employed to monitor thecontamination around a different tool cluster; another conversion moduleis employed to monitor the contamination level within a chemical storagecabinet to provide early indication of chemical spills.

In another implementation, shown in FIG. 6, the converter-detectorinstrument is constructed as a mobile leak detector. The mobile unit ismoved to selected regions of the fabrication facility to seek possibleareas of contamination leaks. By following an escalatingbasic-nitrogen-compound concentration trend, the mobile unit localizesthe source of the contamination.

In another embodiment, illustrated in FIG. 7, the system is combinedwith a multi-point sampling system of an array of sensors to monitor theoperating status of a track, including temperature, temperature of thehot plate, time on the chill plate, exposure time, etc. A totalbasic-nitrogen-compound detector monitors process contaminants, such asthe concentration of an adhesion promoter, e.g., hexamethyldisilozane(HMDS), in a gas sample during the coating stage when photoresist isapplied to the semiconductor wafers. The wafers are then sent to thestepper for exposure and subsequently brought back to the track fordeveloping. During this stage, another, or the same, totalbasic-nitrogen-compound detector monitors the concentration of anotherpossible internally-processed chemical contaminant, such astetremethylammonium hydroxide (TMAH), employed in the developing stage.

The system enables, in its total basic-nitrogen-compound reading, thesimultaneous detection of NMP and ammonia, which previously weretypically monitored with separate detectors. The system also enablesdetection, in its total basic-nitrogen-compound reading of other basicnitrogen compounds that are known to be harmful to the photolithographyprocess, such as morpholine, diethylamine ethanol, and cyclohexylamine,agents which are commonly used to inhibit corrosion in high-humidityregions. Basic nitrogen compounds from the facility cafeteria,especially seafood, are also included in the detection as well as basicnitrogen compounds from the breath of the facility workers, which cancreate high levels of basic-nitrogen-compound contamination, dependingupon diet and smoking habits. As has been explained, the system, asillustrated, converts substantially all such airborne basic nitrogencompounds to a common detectable compound, which it detects to indicatethe level of hydrogen-bonding contaminants. If high concentrations ofthe contaminants are detected, by grab sampling techniques, the exactsources of the contamination can be determined and remedied.

Another advantageous aspect of the system is its adaption to thecertification process of clean rooms. Heretofore, during thecertification process, each individual molecular base present in theclean room was typically detected by a separate detector. Theconcentrations were summed providing a number indicating the total baseloading in the clean room. For instance, if three bases were present,each with a concentration of 10,000 parts per trillion (ppt), the cleanroom rating would be MB30,000 (or 30,000 ppt). the present inventionsolves the problem of detecting individual bases by providing the totalbase loading within a clean room with a single reading.

The following embodiments of the invention are particularly effective inproviding an accurate determination of total basic nitrogen compounds atlow concentrations, for protection of the base-sensitive processes.These embodiments include a primary channel and a plurality of scrubbingchannels for delivering gas samples from a source to the converter anddetector. Each of the scrubbing channels includes a removal device forremoving basic nitrogen compounds from the gas samples to producereference samples. In some embodiments, the removal device isprocess-specific, meaning that it removes only those basic compoundsthat are of interest for a specific process. For example, in anembodiment specifically designed for use with semiconductor fabricationtools, the removal device is coated with photoresist to remove from thegas sample those compounds that will interfere with photolithographicdeposition and development. The primary channel, meanwhile, deliverstarget gas samples from the same source to the convertor withoutscrubbing.

Because the scrubbing channels produce a reference sample that is freeof substantially all of the gas molecules corresponding to the class ofbasic nitrogen compounds of interest, the reference sample provides abaseline for canceling the effects of background, nitrogen-containingcontaminants which might otherwise contribute to the NO concentrationdetected by the chemiluminescent detector in the target gas sample.

Referring to FIG. 8A, a sample delivery train, including a conversionmodule 6, is shown for determining the total basic-nitrogen-compoundconcentration in a gas sample using an internal reference for zero air(i.e., air that is free of basic nitrogen compounds). The sample entersthrough a selection valve 114 (e.g., a multi-position valve manufacturedby Vici Co., Houston, Tex., U.S.A.), which creates an open flow pathfrom one of the sampling ports 112. From the selection valve 114, thesample is directed to the conversion module 6, which splits eachincoming sample into two parts, channels A and B, and directs each partseparately, via a multi-way valve 122, to a thermal catalytic converter124 followed by a chemiluminescent detector 126. Alternatively, a pairof three-way valves (illustrated as valves 122 a and 122 b in FIG. 8Cand connected as shown) can, in practice, be substituted for themulti-way valve 122. Another valve 123 is provided upstream from thescrubber system 120 to help direct the flow of the gas sample intochannels A and B and to prevent backflow through the channels.

Channel A, which serves as a “scrubbing channel,” directs part of thesample through a basic-nitrogen-compound scrubber system 120 and then tothe valve 122, while channel B, referred to as the “primary channel,”directs part of the sample directly to valve 122. The term, “scrubber,”as used in this document is intended to refer to any device that iseffective for removing basic nitrogen compounds, or a device thatotherwise treats the basic nitrogen compounds in such a way that theyhave no effect on the response of the detection system being employed.

The scrubbed and unscrubbed parts of the original sample from channel Aand B, respectively, proceed from valve 122 through the thermalcatalytic converter 124 to the chemiluminescent detector 126 astime-separated samples in a single conduit. The difference between theresponse of the chemiluminescent detector 126 to samples from channel Aand B reflects the total basic-nitrogen-compound concentration in theoriginal sample of gas.

It has been determined that the sensitivity of basic-nitrogen-compounddetection improves as the operating pressure of the chemiluminescent NOconcentration measurement decreases. Accordingly, the chemiluminescentNO detector 126 operates at a pressure of 125 millibar or less, byco-action of vacuum pump 128 and an upstream pressure reducer 118, toachieve a low noise level and to achieve a detection sensitivity of 1part per billion (ppb), preferably 0.5 ppb, or better. Though the idealoperating pressure in the detector 126 is a function of the gas flowrate through the detector in any given application, a preferred pressurelevel in the detector is generally about 40 mbar (about 30 Torr).

The conversion module 6 initially takes a sample of gas from one of thesampling ports 112 via selection valve 114 and channels it through oneof the sample lines 116. Sample lines 116 are tubes formed of Teflon®PFA from E.I. DuPont de Nemours of Wilmington, Del., U.S.A..Alternatively, sample lines 116 can be stainless steel tubes coated withCVD-coated silica (e.g., Silcosteel™ from Restek Corp., Bellefonte, Pa.,U.S.A.). Sample lines coated with silica are nonporous and nonreactiveand, therefore, have little effect on ammonia and organic amines passingthrough the tubes. In contrast, commonly-used tubing of PTFE isrelatively porous and tends to emit hydrogen fluoride, a strong acid,which reacts with ammonia and amines, interfering with the measurementof total basic nitrogen compounds. Silica may be deposited onto thestainless steel tubing using chemical vapor deposition. In certainapplications, sample lines comprising glass are used. The glass samplinglines may be reinforced with epoxy.

In certain advantageous embodiments, sample lines 116 are heatedsubstantially along their total length to approximately 50° C. usingelectrical heating lines (see, e.g., U.S. Pat. No. 3,727,029,incorporated herein by reference). This reduces the tendency for basicnitrogen compounds to deposit on the walls of the tubing (reducesbasic-nitrogen-compound-sticking coefficient) and thus reduces sampleline contamination of the alternating sample gas slugs.

In this context, the combination of heating the sample lines along withusing a silica coating on the lines is advantageous because it reducesboth contamination of the gas sample and deposition of gas samplecomponents. Nevertheless, the practice of heating sample lines of othercompositions, by itself, can provide a beneficial effect. Heating ispreferably accomplished using electrical resistance wire incorporated inthe wall of the sample line or otherwise disposed in thermal contactwith it.

Selection valve 114 (e.g., a multi-position valve manufactured by Vici)enables samples from different locations to be channeled into a singleconversion module 6. The gas sample then flows through channel A or B tomulti-way valve 122. After exiting valve 122, the gas sample passesthrough a pressure reducer 118 (e.g., a flow restrictor, such as acapillary glass tube or a small orifice), where the pressure drops fromatmospheric pressure on the upstream side of pressure reducer 118 toapproximately one-tenth atmospheric pressure on the downstream side.This pressure drop is maintained by vacuum pump 128 positioneddownstream of detector 126. The pressure reducer 118 is preferablypositioned downstream of the scrubber system 120 because the scrubbersystem 120 operates with greater effectiveness at higher pressures.Preferably, the pressure reducer 118 comprises a calibrated glasscapillary heated to 50° C. to reduce the basic-nitrogen-compoundsticking coefficient. The pressure reducer 118 can be made from tubes ofglass, ceramic, stainless steel, quartz, or stainless steel with aninterior plated with gold or other inert material. The low pressure ofthe samples-created by pressure reducer 118 and vacuum pump 128, inaddition to enabling high sensitivity detection, reduces the responsetime for measuring total basic-nitrogen-compound concentration. This isbecause the samples travel through the delivery train rapidly; forinstance, in the system described, valve 122 may be shifted betweenchannels A and B every 10 seconds in normal operation.

Auxiliary conduit 116 a is connected at one end to valve 122, At itsopposite end, auxiliary conduit 116 a is connected to the vacuum pump128. Auxilary conduit 116 a includes a pressure reducer 118 a to limitflow therethrough. During operation, valve 122 connects the non-selectedchannel A or B to the auxiliary conduit 116 a to maintain a gas flowthrough the non-selected channel. In this way substantially-steady-stateflow conditions can be maintained in the scrubber system 120, and freshsample is immediately available to the converter 124 upon actuation ofvalve 122. Further, the steady-state flow of gas through the channelsreduces the incidence and magnitude of pressure spikes that can occurupon switching and thereby produce erratic instrument readings.

The components of a scrubber system 120 of this invention areillustrated in FIG. 8B. The scrubber system 120 includes three scrubbers121 connected in parallel to the sample line 116. The scrubbers 121 ofchannel A are constructed to selectively remove from the gas sample thetotality of the basic nitrogen compounds to which a photolithographic orother process being guarded is sensitive, yet their construction andfunction are nevertheless such as to not affect othernitrogen-containing compounds. Each scrubber 121 is preferably asolid-state scrubber, comprising an ion exchange medium with activesulfonic or carboxylic groups. The ion exchange medium is substantiallynon-hygroscopic and nonporous. The medium can be in the form, forexample, of either fibers or a resin. The scrubber may also comprise anymaterial that preferentially binds the airborne molecular bases (e.g.,photoresist-coated substrates, weak acid-coated substrates, strongacid-coated substrates, ion exchange materials, or chemically-treatedactivated carbon and molecular sieves). The properties of the activemedia can be chosen to optimize the selectivity of the detectionprocess. Further still, the scrubber can be a chemical gas filtermedium, along the lines of the air filter that supplies “zero air” inthe previously described embodiments.

A strong cation exchange medium is preferred as the scrubber substancein the scrubbers 121 of FIG. 8B. The medium withinbasic-nitrogen-compound scrubber 121, being a strong acid, removesmultiple molecular bases, such as ammonia and other basic nitrogencompounds. A strong cation exchange medium ensures removal of bothstrong and weak molecular bases, and is suitable for photoresistlithography techniques that are sensitive to both strong and weak bases.For techniques sensitive only to strong bases, a weaker acid ionexchange medium may be employed, so that weak bases are not removed andoccur equally in the scrubbed and unscrubbed samples.

Nevertheless, even when a strong cation exchange medium is used,chromatic differences in removal efficiency will develop over time. Astrong base, such as ammonia (NH₃), will be tightly bound to the cationexchange medium and, once it is removed from the gas sample, will likelyremain fixed to the scrubber. A weak base (typically, one with a highmolecular weight, such as NMP), however, will tend to gradually driftthrough the scrubber, repeatedly binding to and releasing from thecation exchange medium. Gradually, over a matter of hours, NMP will workits way through and out of the scrubber. When a tube that is ½ inch indiameter and 2 feet in length is used with a flow rate of 1 liter perhour, NMP will pass through the scrubber in about 15 hours. Bases withstrengths between that of ammonia and NMP will gradually drift throughthe scrubber, albeit at rates slower than that of NMP.

Nine three-way valves 125, 125′ and 125″ act as flow controllers,controlling the flow of sampled gas and purge gas through the scrubbers121. The valves 125, 125′, 125″ in turn, are controlled by a controlsystem 130. When a pair of valves 125, 125′ on opposite sides of thesame scrubber 121 both open channels to the sample line 116, sampled gasflows through that scrubber 121, with the scrubber 121 filtering basicnitrogen compounds from the sampled gas.

In one embodiment, the control system 130 selectively opens the channelsto the sample line 116 in a single pair of opposing valves 125, 125′ ata given moment. The control system then cyclically redirects the flowfrom the sample line 116 to each of the scrubbers 121, in sequence. Theduration of flow (which is still received as a series of pulses,alternating with channel B, shown in FIG. 8A) through each scrubber 121is limited to less than the time required for a high-molecular-weightcontaminant to pass through the scrubber 121. Preferably, the gas flowis shifted among scrubbers 121 about every 2 to 3 hours. After the flowhas been redirected from a scrubber 121, the control system 130 thensends a signal to respective valves 125, 125′ and 125″ to open a channelfrom the purge line 127 through the respective scrubber 121 and out thepurge line exhaust 127′, illustrated on the left side of FIG. 8B. Thepurge line 127 is supplied by a source of compressed purge gas(typically, air from which bases have been filtered), which flowsthrough valves 125″ and 125′ and then to scrubber 121, so that the purgegas flows through the scrubber 121 in a direction opposite to the priorflow of the sampled gas. The purge gas is directed through each scrubber121 via its respective valves 125, 125′ and 125″ in a sequence followingthat of the sampled gas flow and also for a period of about 2 to 3hours. Accordingly, the purge gas cleanses each scrubber 121 of basicnitrogen compounds removed from the sampled gas. Consequently, a cleanscrubber 121 is always available when the flow of sampled gas isredirected.

In addition to being connected to valves 125′ and to the purge line 127,valves 125″ are also connected to an exhaust line 129. In oneembodiment, the control system 130 regulates valves 125, 125′ and 125″to maintain a flow of sampled gas through each of the scrubbers 121 atall times, except during purging. The flow through only one of thesescrubbers 121 is directed back to the sample line 116, however. Sampledgas flowing through other, inactive scrubbers 121 is directed by thevalves 125′ and 125″ into the exhaust line 129, which removes thesampled gas from the system. The purpose of maintaining gas flow through“inactive” scrubbers is to keep “inactive” channels conditioned so thatwhen the gas flow is redirected to a purged scrubber 121, the detectorreadings are less likely to be skewed by a momentary spike due to theneed to flush old gas from the newly-activated scrubber 121 after ashift.

The sample in channel A, upon exiting scrubber system 120, containsessentially no detrimental molecular bases; i.e., total concentration ofobjectionable basic nitrogen compounds is approximately zero. Thisbasic-nitrogen-compound-free sample becomes the reference sample forpurposes of measuring the total basic-nitrogen-compound concentration inthe unscrubbed sample from channel B. The scrubbers, however, do notaffect neutral or acidic nitrogen-containing compounds. Otherwise, afalse signal would be produced because those same compounds remain inthe target sample.

In other embodiments, the reference gas and the target gas may besampled from different inputs. In these embodiments, it is desirable toconstruct channels A and B to have the same pressure drop from the inputlocation to the converter input. Referring to FIG. 8C, separate pressurereducers 118 a, 118 b may be used in channels A and B. To ensure thatthe pressure drop in each channel is approximately the same, pressurereducer 118 a in channel A contains a larger orifice than pressurereducer 118 b to compensate for the affect of basic-nitrogen-compoundscrubber system 120 on the pressure in channel A. Thus, the pressuredrop between scrubber system 120 and three-way valve 122 a is equivalentto the pressure drop between pressure reducer 118 b and three-way valve122 b.

Valves 122 a and 122 b allow the basic-nitrogen-compound-free samplefrom channel A and the unscrubbed sample from channel B to be directedto thermal catalytic converter 124 alternately, in rapid sequence.Operating the delivery train at 125 millibar pressure, for example,enables the three-way valves 122 a and 122 b to jointly switch the flowof sample gas between channel A and channel B several times per minute,to enable averaging of a number of readings, if desired, within a shortmonitoring interval, for instance ten minutes or less. While eithervalve 122 a or valve 122 b directs a gas sample from one channel throughthe thermal catalytic converter 124 and detector 126, the other valve122 a or 122 b maintains continuous flow through the other channel bydirecting gas that passes therethrough into auxiliary conduit 116 a.

Thermal catalytic converter 124 (e.g., as manufactured by ThermoEnvironmental Instruments Inc.) converts basic nitrogen compounds ineach gas sample to nitric oxide (NO) by thermal oxidation. A suitablecatalytic converter 124 is illustrated in FIG. 8D. The converter 124comprises a reaction chamber 150 that may or may not contain a catalyticelement 152 (e.g., platinum and/or palladium), a heating element 154 toheat the reaction chamber, and a thermocouple 156 connected to powercontrol relay 158 which regulates the temperature of the reactionchamber. Since any given sample from channel B may contain a variety ofbasic nitrogen compounds (such as morpholine, diethylamino ethanol,ammonia, and normal methyl pyrrolidinone), thermal catalytic converter124 must have a high conversion efficiency for many types of basicnitrogen compounds. To achieve a high conversion efficiency (85-100%)for a broad range of basic nitrogen compounds, a stainless steel surfaceheated to 900° C. is used as the catalyst. Alternatively, a metal oxidesurface can be used. The gas sample is oxidized as it passes over theheated surface resulting in the conversion of basic nitrogen compoundsto NO. If the gas sample lacks oxygen for oxidation, oxygen gas shouldbe supplied to the converter.

In other embodiments, the converter may perform photocatalysis, whereinthe bonds of nitrogen-containing molecules are split with ultravioletlight to free nitrogen atoms for oxidation to thereby form NO. Theappropriate conversion technique is determined by the desiredapplication, taking into account cost and conditions of use.

In some applications, gas samples entering thermal catalytic converter124 may contain molecules that are not basic nitrogen compounds, butwill nonetheless be converted into NO. For example, compounds such asNF₃, HCN and CH₃CN are not basic nitrogen compounds, yet if present inthe gas sample, will be converted to NO in thermal catalytic converter124. These compounds will not affect the total basic-nitrogen-compoundconcentration calculation, however, because the basic-nitrogen-compoundscrubbers 121 do not retain these compounds since they are not bases. Assuch, samples from channel A and channel B contain equal amounts ofthese non-basic compounds, and thermal catalytic converter 124 convertsthese compounds to NO equally for channel A and channel B, thuscanceling out any effect.

Likewise, where the process to be monitored is sensitive only to strongbases, scrubbers that include a medium, e.g., a weak acid, that removesonly the various strong-base nitrogen compounds are selected. In thiscase, since the weak-base nitrogen compounds will be present in bothchannels, their presence does not affect the response of the system.Alternatively, basic nitrogen compounds can be differentiated on thebasis of strength by using converters that convert bases of differentstrengths with different efficiencies.

Converted samples exit thermal catalytic converter 124 and enterchemiluminescence detector 126 (e.g., as manufactured by ThermoEnvironmental Instruments, Inc.). The detector 126 employschemiluminescence for NO detection. Typically, the maximum signal fromchemiluminescence detector 126 is achieved at a pressure of about 65Torr and at a flow rate of about 1.5 liters per minute under theoperating conditions described above. Detector 126 operates atapproximately 125 millibar, suitable for detection of low concentrationsof basic nitrogen compounds. This high sensitivity affords detection oftotal basic-nitrogen-compound concentrations of less than 1 ppb,preferably less than 0.5 ppb.

Detector 126 employs chemiluminescence for NO detection. For thispurpose

NO+O₃→NO₂*+O₂,

NO is caused to react with ozone generated by an internal ozonegenerator in a reaction chamber of the detector 126. This produceselectronically-excited NO₂ molecules (NO₂*), which, in returning to theground state, emit photons, hν, that are detected by anappropriately-cooled photomultiplier tube. The reaction is given by theexpression,

NO₂*→NO₂ +hν.

To achieve the needed sensitivity for current DUV photolithographicprocesses with presently-available photomultipliers, the photomultipliertube is cooled at least to −5° C. To achieve sensitivities required fornext-generation fine-resolution DUV photolithography in semiconductormanufacturing, the tube is cooled to −15° C. by an associatedthermoelectric cooler. Moreover, since there is significant variation inthe sensitivity of photomultiplier tubes produced by the samemanufacture, the analyzer sensitivity is further increased by testingand choosing an optimum photomultiplier tube for the performancerequired. In other implementations, NO is detected by calorimetricmethods using devices available from, e.g., Tytronics, Inc. of Bedford,Mass.; other methods that are based upon continuous in-line sampling mayalso be used.

The signal from the photomultiplier is converted into time-based NOconcentration values by a control system 130, and then the totalbasic-nitrogen-compound concentration of the gas sample from theselected sampling point is determined, e.g., by appropriately averagingand differencing the values. The total basic-nitrogen-compoundconcentration for the gas sample equals or is proportional to(accounting for incomplete NO conversion of basic nitrogen compounds)the difference between the NO concentration of the unscrubbed samplefrom channel B, e.g., at time t₁ (or the average of NO concentrations attimes t₁, t₃, . . . t_(n−1), n being an even number) and the NOconcentration for the basic-nitrogen-compound-free sample from channelA, e.g., at time t₂ (or the average of NO concentrations determined attimes t₂, t₄, . . . t_(n), n being an even number).

In an alternative embodiment, the detector 126 comprises a plurality ofdetection devices, with a separate detection device being allocated toeach channel.

Additional channels may be used to provide further analysis of thecomponents of the sampled gas. For example, referring to FIG. 8E, threechannels (CH1, CH2 and CH3) may be used to determine the componentconcentrations of NO, NO₂ and other non-basic nitrogen-containingcompounds which are convertible to NO, and the total basic nitrogencompounds in a gas sample. The following discussion focuses on thevarious classes of nitrogen-containing compounds in the sampled gas. NO,NO₂, basic nitrogen compounds, and other N-containing compounds reachthe NO detector through CH1; and the NO detector produces a signalrepresentative of the NO concentration in the sampled gas. NO (includingNO converted from NO₂ and other convertible N-containing molecules) andnon-convertible N-containing molecules reach the NO detector throughCH2; and the difference between the detector signals for CH1 and CH2provides a measure of the concentration of NO₂ and other non-basic,nitrogen-containing compounds which are convertible to NO in the sampledgas. NO (including NO converted from NO₂, basic nitrogen compounds andother convertible N-containing molecules) and non-convertibleN-containing molecules reach the NO detector through CH3; and thedifference between the detector signals for CH2 and CH3 provides ameasure of the total basic-nitrogen-compounds concentration in thesampled gas.

In some implementations, variations that occur in the ambient NO and NO₂concentrations may affect the accuracy of the totalbasic-nitrogen-compound concentration measurement. In such cases wherethe disturbance warrants, an algorithm is used to minimize the effectsof such fluctuations by calculating the total basic-nitrogen-compoundconcentration based on a moving average for NO concentrations fromchannel A and channel B. For example, each NO concentration measurementis added to the previous consecutive measurements and divided by thetotal number of measurements made at that point in time. This movingaverage calculation may be represented by the following algorithm:

Moving NO Average=(X 1+X 2+ . . . +Xn)/n,

where X equals the NO concentration at a given time and n equals thetotal number of NO measurements made. The moving average calculation maybe reset periodically to avoid the weighting of out-of-datemeasurements. In another algorithm, a selected number of values areadded together to provide an initial average value and thereafter theoldest value is dropped from the average as the newest measured valve isadded to it.

As shown in FIGS. 8A and 8B, control system 130 (e.g., a computer), inaddition to collecting and analyzing data received fromchemiluminescence detector 126, controls selection valve 114, valve 122(or valves 122 a and 122 b, as shown in FIG. 8C), and scrubber-systemvalves 125, 125′ and 125″. Selection valve 114 is controlled by controlsystem 130 to channel samples from multiple sampling ports 112 into theconversion module 6 in a selected order. Valve 122 is controlled toswitch between channel A and channel B on the basis of settling times(i.e., the time required for the measured NO value to reach equilibriumfollowing a shift). Preferably, multiple switching cycles are employedfor a given sample line and the measurements are averaged, or a runningaverage is employed, to produce a reliable measure of total basicnitrogen compounds, as has been described.

Referring to FIG. 9, in another embodiment, conversion module 6 employsan additional valve 140 at the branching point of channels A and B.Valve 140, in conjunction with valve 122, isolatesbasic-nitrogen-compound scrubber system 120 and thus prevents thepossibility of back-flow diffusion. The amount of sample that isdirected by valve 140 to channel A and channel B is controlled bycontrol system 130. In this embodiment, valve 122 is also connected tothe vacuum pump by an auxiliary conduit 116 a (which, again, isconnected, in parallel with sample line 116, to a common vacuum pump) todraw gas through the non-selected channel so that substantiallycontinuous flow conditions are maintained through both channels A and Bat all times. Valve 140 is also connected to sample lines 116 b and 116c, which are connected, in parallel, upstream from channels A and B.Sample lines 116 b and 116 c include respective pressure reducers 118 band 118 c, which restrict the flow of sample gas to different degrees.When the sample gas is flowing through channel B, valve 140 selects thesample line with the more-restrictive pressure reducer (in this example,sample line 116 c) as the source of that gas. The resistance of themore-restrictive pressure reducer is selected to offset the inherentflow restriction generated by the scrubber system 120. While channel Bis selected, valve 140 connects sample line 116 b, which has theless-restrictive pressure reducer, to supply a flow of gas samplethrough channel A and on through auxiliary sample line 116 a.

Referring to FIG. 10, in another embodiment, a separate and distinctchannel for producing a reference of zero air is operated parallel withanother channel for determining the total basic-nitrogen-compoundconcentration. Sampling point selection valves 114 a and 114 b, forchannels 1 and 2, may be gauged and arranged to simultaneously samplethe same location. Channel 1 produces a reference of zero air bydirecting a sample to basic-nitrogen-compound scrubber system 120 andthen to a thermal catalytic converter 124 a and chemiluminescencedetection device 126 a. At the same time, channel 2 directs a sample toa thermal catalytic converter 124 b and a chemiluminescence detectiondevice 126 b. The NO measurements from channel 1 and 2 are madesimultaneously and then compared. This embodiment eliminates the effectof fluctuations in ambient NO and NO₂ concentrations by determining theactual NO and NO₂ concentration at the same time as the total NOresponse is being measured.

Control system 130′ calculates the total basic-nitrogen-compoundconcentration based on the differences between the two readings from thedetectors 126 a, 126 b. A calibration system (not shown) is employed tocompatibly zero the instruments (e.g., to accommodate variations in theconverters and detection devices) so that they can operate together. Acorrection factor based upon, e.g., computer look up of an experiencetable, can be employed for calibration purposes. A calibration routinecan be conducted periodically, and drift trends can be measured andstored to create a dynamic correction algorithm.

This arrangement eliminates the possibility of noise from variations inambient NO and NO₂ concentrations because the instantaneous value of theNO and NO₂ concentration is always known and does not change during acalculation cycle. This system, in effect, reduces the time betweenambient NO and NO₂ measurements to zero, which solves the fundamentalproblem of fluctuations in NO and NO₂ concentrations during a singlecalculation cycle.

The various embodiments may be implemented in a number of useful ways.The parameters of the basic-nitrogen-compound scrubbers are selected toremove only those basic nitrogen compounds that affect a given process.In one advantageous example, beds of photoresist-coated beads are usedas the scrubbers, the photoresist material being selected to correspondto the photoresist material being employed in the process beingmonitored. Thus, the scrubbers remove those basic nitrogen compounds towhich the photoresist process is peculiarly sensitive. In an importantcase, the scrubbers used in the detection systems of FIGS. 8-10 areconstructed and arranged to select construction materials for use in abasic-nitrogen-compound-sensitive process (e.g., chemically-amplifiedphotoresist process). In another important case, the detection system ofFIGS. 8-10 is connected to monitor the performance of abasic-nitrogen-compound gas filter system used to filter gas in a DUVstepper, scanner or coat/develop track. In another important case, thedetection system of FIGS. 8-10 is connected to monitor the totalbasic-nitrogen-compound concentration inside a DUV stepper, scanner, orcoat/develop track. In another important case, the detection system ofFIGS. 8-10 is connected to monitor cleanroom concentration of totalbasic-nitrogen-compound concentration inside a DUV stepper, scanner, orcoat/develop track.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A detection system for detecting contaminant gases, comprising: a detector that detects a concentration of an indicator gas in a gas sample; a primary channel that delivers a target gas sample to the detector; a plurality of scrubbing channels for delivering a reference gas sample to the detector, wherein each of the scrubbing channels includes a scrubber for removing basic nitrogen compounds from the reference gas sample; and a converter coupled to the primary channel and to the scrubbing channels for converting gaseous nitrogen compounds in the gas samples into the indicator gas before the gas samples reach the detector.
 2. The detection system of claim 1, further comprising a purge system coupled to at least one of the scrubbers for purging reversibly-bound basic nitrogen compounds from the scrubber.
 3. The detection system of claim 2, wherein the scrubbers include a cation exchange medium.
 4. The detection system of claim 2, wherein the scrubbing channels are connected in parallel.
 5. The detection system of claim 4, further comprising a flow controller positioned for selectively controlling which of the scrubbing channels the sampled gas can flow through to the converter.
 6. The detection system of claim 5, wherein the flow controller is governed by a control system that is programmed to transfer the flow of the reference gas sample reaching the detector from a scrubbing channel with a contaminated scrubber to a scrubbing channel with a purged scrubber and to then direct a purge gas through the contaminated scrubber.
 7. The detection system of claim 6, wherein the control system is programmed to transfer the flow of the reference gas sample away from one of the scrubbing channels and to purge the scrubber of that scrubbing channel before a weak-base nitrogen compound can penetrate through the scrubber.
 8. The detection system of claim 7, wherein the control system is programmed to alternately transfer the flow of a gas sample between the primary channel, where the gas sample becomes the target gas sample, and one of the scrubbing channels, where the gas sample becomes the reference gas sample.
 9. The detection system of claim 2, wherein the scrubbers include photoresist-coated beads.
 10. The detection system of claim 2, further comprising a pressure reducer located between the detector and the scrubbers.
 11. The detection system of claim 2, wherein scrubbers are excluded from the primary channel.
 12. A method for monitoring basic-nitrogen-compound contamination in sampled gas, comprising the steps of: passing a first reference gas sample through a first scrubber to remove basic nitrogen compounds from the first reference gas sample; passing a second reference gas sample through a second scrubber to remove basic nitrogen compounds from the second reference gas sample; purging the first scrubber to remove reversibly-bound basic nitrogen compounds while passing the second reference gas sample through the second scrubber; passing the first and second reference gas samples through a converter which converts gaseous nitrogen compounds in the reference gas samples into an indicator gas after the reference gas samples are passed through their respective scrubbers; passing the first and second reference gas samples through a detector which detects the concentration of the indicator gas in the reference gas samples after the reference gas samples are passed through the converter; passing a target gas sample through the converter which converts gaseous nitrogen compounds in the target gas sample into the indicator gas; passing the target gas sample through the detector which detects the concentration of the indicator gas in the target gas sample after the target gas sample passes through the converter; and determining a total basic-nitrogen-compound contamination concentration by comparing the detected concentration of the indicator gas in the target gas sample with the detected concentration of the indicator gas in at least one of the reference gas samples.
 13. The method of claim 12, wherein a common detection device alternately detects the concentration of the indicator gas in the reference gas sample flowing through the first scrubber and the concentration of the indicator gas in the reference gas sample flowing through the second scrubber.
 14. The method of claim 12, further comprising the step of alternately purging the first and second scrubbers while maintaining substantially uninterrupted flow of the gas samples to the converter and to the detector.
 15. The method of claim 12, wherein the gas samples are taken from a photolithography tool cluster.
 16. A photolithography system, comprising: a photolithography tool cluster; a detector that detects a concentration of an indicator gas in gas samples from the photolithography tool cluster; a primary channel that delivers a target gas sample to the detector; a plurality of scrubbing channels for delivering a reference gas sample to the detector, wherein each of the scrubbing channels includes a scrubber for removing basic nitrogen compounds from the reference gas sample; at least one converter coupled to the primary channel and to the scrubbing channels for converting gaseous nitrogen compounds in the gas samples into the indicator gas; and a control system that controls the flow of the gas samples among the primary channel and the scrubbing channels.
 17. The detection system of claim 16, further comprising a purge system coupled to each of the scrubbers for purging removed basic nitrogen compounds from each of the scrubbers.
 18. The detection system of claim 17, wherein the scrubbers include ion exchange media. 